1. Field of the Invention
This invention relates to a bi-directional pressure balanced valve and, more specifically, to such a valve which includes an axial opening through a main valve disc assembly which facilitates effective balanced operation of the valve and has been improved to prevent velocity pressure created by fluid flow from beneath the valve seat from interferring with effective balanced closure.
2. Prior Art of the Invention
There has heretofore been provided a large bi-directional pressure balanced valve of the type disclosed in U.S. Pat. No. 3,888,280 for use in high pressure fluid lines. The balanced configuration enables fail-safe closure of the valve regardless of the direction of the flow therethrough by a plurality of springs if an opening force of a pneumatic motor assembly is no longer energized to maintain the valve in an opened position. If the valve were not effectively balanced, the force needed to close the valve under adverse conditions, such as during a sudden line break, would require a significantly larger spring configuration which would be physically impractical to provide. To insure against valve damage during closure, a dashpot assembly controls the rate of closure by the springs to insure that it is maintained within an acceptable safe limit.
Although the valve of U.S. Pat. No. 3,888,280 has generally provided a satisfactory means for controlled prevention of fluid flow in either direction, it has been found that excessive pulldown forces can be generated when a large, 32-inch diameter valve is closed under a particularly demanding flow condition. It might, for example, be necessary to close the valve against fluid flow from below the seat at about 1,000 p.s.i. inlet pressure with the outlet at zero pressure, as might occur with a line rupture. An analysis based on tests of a smaller model and using accepted engineering and analytical methods has indicated that the pulldown force generated on such a large valve under these extreme conditions could overload the dashpot and might result in its failure. Specifically, it was found that the 32-inch diameter balanced valve in an opened, lifted position 253/4 inches from the seat would be fully closed by a spring force ranging from 76,000 pounds to 45,000 pounds. However, if the total pressure produced in the dashpot were greater than 5,000 p.s.i., the integrity of the dashpot would be threatened and its ability to prevent valve damage would be impaired. According to the analysis, the undesired pulldown force produced by fluid flow acting on the valve during closure would be the greatest, 98,000 pounds, when the valve is about 19 inches from the seat. When this pulldown force is added to the spring force and resisting frictional forces are subtracted from the total, the expected dashpot pressure would exceed 7,000 p.s.i.
Although a larger and/or heavier dashpot assembly might be designed to satisfy this condition, there are other considerations which make this an unattractive alternative. The space presently provided the dashpot assembly within the operating mechanism for the valve is limited and physically relocating the springs or any other components of the mechanism would require extensive, complicated redesign. Because power plant safety requirements include seismic considertions and the ability of the valve and the pipes to withstand shock, it is also undesirable to increase the mass of the dashpot assembly, and thus the operating mechanism which extends in a cantilevered fashion from the pipes. Therefore, determining the cause of the excessive pulldown forces and providing for their reduction is most desirable.
The rapid fluid flow from beneath the seat produces a velocity pressure which is added to the static pressure of the fluid to produce a higher, stagnation pressure in an axial opening in the main valve disc assembly. The effect of the velocity pressure on the axial opening is then transmitted into the upper chamber of the valve body through the axial opening of the main valve assembly and unintentionally augments the static pressure needed to provide the desired balanced operation. It has been assumed that the resulting stagnation pressure acting on top of the main valve disc assembly which creates these excessive pulldown forces is not sufficiently opposed by a lesser total pressure from beneath the main disc which acts directly thereon because of the inclination of the inlet pipe, and thus the direction of fluid flow, with respect to the main disc. The velocity pressure apparently elevates the stagnation pressure within the isolated upper chamber of the body despite efforts during main valve closure to urge an auxiliary valve disc toward a closed position which could prevent flow through the axial opening.